PET-MRI

PET-MRI combines positron emission tomography with magnetic resonance imaging in one scanner, acquiring both studies together or one soon after the other. The MRI provides high soft tissue contrast and functional information without ionising radiation, and the PET provides molecular information.

Combining them gives one set of registered images in a single visit.

The PET tracers match those used for PET-CT: fluorodeoxyglucose, gallium-68 and fluorine-18 PSMA agents, somatostatin receptor tracers, and brain tracers including amyloid, tau and dopamine agents. Doses match, or are slightly lower.

Attenuation correction uses MRI sequences that separate air, lung, fat and soft tissue (a fat water separation sequence), as there is no CT. The effective dose is about 3 to 7 mSv, 30 to 80% less than PET-CT, because there is no CT dose.

A whole body study takes 30 to 60 minutes, and organ directed techniques of 20 to 40 minutes are used for the brain, head and neck, pelvis, liver or heart.

The method suits patients who need both PET and MRI information, and those in whom the dose matters, such as children and young adults.

PET-MRI has been shown to be at least as good as PET-CT for most cancers, and better for brain tumours, head and neck cancer, pelvic tumours, liver metastases and bone marrow disease. It is not routinely used for lung nodules.

Indications.

• Children and young adults with lymphoma, sarcoma and brain tumours, when radiation dose is a concern.
• Brain: tumour grade and extent, epilepsy surgery planning (focus localisation), and dementia (amyloid or tau PET with structural MRI).
• Head and neck cancer: local extent and nodal disease.
• Pelvic cancer: prostate (PSMA PET-MRI), rectal, cervical, endometrial and bladder cancer, with local staging and nodal assessment.
• Liver lesions: metastases from colorectal and neuroendocrine tumours, and hepatocellular carcinoma.
• Neuroendocrine tumours (receptor PET with MRI of the liver).
• Breast cancer: dedicated PET-MRI for staging and response.
• Cardiac disease: sarcoidosis, myocarditis, amyloidosis and cardiac tumours (tissue characterisation with PET).
• Inflammatory disease: large vessel vasculitis, inflammatory bowel disease (small bowel), and musculoskeletal infection.
• Cancer of unknown primary and repeated staging in patients needing frequent imaging.

Contraindications and precautions.

• MRI safety rules apply: pacemakers and implants not labelled as conditional, intracranial metal, cochlear implants, and some drug pumps.
• Claustrophobia, and inability to lie still for a long study.
• Pregnancy: avoid PET tracers unless essential.
• Breast feeding: the advice depends on the tracer used.
• Gadolinium contrast: renal precautions apply, and it is used only when needed.
• The fasting, glucose and rest rules for fluorodeoxyglucose PET also apply.
• Large body habitus exceeding the bore diameter.
• Metal in the body, including dental work, causes artefacts on MRI-based attenuation correction.

Preparation and technique.

The preparation matches that for the tracer used, together with MRI screening and removal of metal. The patient is injected and rests for the usual uptake time (about 60 minutes for fluorodeoxyglucose), then lies in the scanner with surface coils.

The MRI attenuation sequence is taken first. PET data are collected at the same time as the diagnostic MRI sequences. A whole body study uses coronal T1, T2 and diffusion sequences, with extra sequences of the organ of interest. Contrast enhanced sequences are added for the liver, pelvis or brain.

The images are fused and reviewed together. MRI-based motion correction is used for the heart, lung bases and liver.

Normal appearances.

Normal distribution of the tracer matches that in PET-CT, and the MRI shows normal anatomy and signal for each organ. Physiological uptake in the brain, heart, kidney and bladder matches PET-CT, and brown fat can be seen on PET with T1 and T2 correlation.

Interpretation.

• Brain tumour: MRI shows the extent, oedema and enhancement, and PET shows the metabolically active tumour, which directs biopsy and radiotherapy. PET separates recurrence from radiation necrosis in many cases.
• Epilepsy: focal hypometabolism that matches an MRI abnormality (hippocampal sclerosis, cortical dysplasia) supports surgery.
• Dementia: amyloid or tau PET with MRI atrophy patterns and vascular lesions.
• Head and neck: MRI gives local extent and PET gives nodal and distant disease.
• Pelvic tumours: MRI shows local invasion and PET shows nodal and bone metastases. PSMA PET-MRI improves detection of prostate cancer in the gland and the bed after surgery.
• Liver: MRI with diffusion and contrast shows small metastases, and PET gives the whole body distribution.
• Bone marrow: PET and diffusion MRI show marrow disease, with better detection than CT.
• Cardiac: late gadolinium enhancement with fluorodeoxyglucose uptake marks active inflammation.
• Fusion allows measurement of uptake in the lesions defined on MRI, and of apparent diffusion coefficients together with uptake.
• Attenuation correction errors can make lesions in bone and lung appear to have lower uptake. These are known and corrected for in reading.

Critical values and findings needing immediate action.

• Cord compression or large mass effect from tumour: urgent neurosurgical or oncology review.
• Large liver or lung lesions causing obstruction: specialist review.
• Active myocardial inflammation with arrhythmia: cardiology review.
• Intracranial haemorrhage or acute stroke found on the MRI: urgent neurology care.
• Pacemaker or implant found unexpectedly in the scanner: stop and follow MRI safety procedures.

Limitations and artefacts.

• MRI-based attenuation correction cannot model bone and lung as well as CT, and leads to underestimated uptake in bone lesions and near metal.
• Lung nodules and small lung lesions are poorly seen on MRI, and CT of the chest is added when the lung is the question.
• Metal implants and dental work cause signal loss and distortion.
• Long scan times raise the risk of motion and make it difficult for children and ill patients.
• Quantitative values differ from PET-CT, and comparison needs the same scanner type.
• Magnetic field strength and the coil set up limit coverage of the whole body in detail.
• Availability is small, and cost is higher than for PET-CT.
• Reader training in both modalities is needed.
• Bowel motion and respiratory motion degrade pelvic and abdominal images.

Special groups.

• Children: valuable because of the lower dose, with sedation or anaesthesia when needed, and a team of paediatric radiology and nuclear medicine.
• Pregnant patients: MRI alone is used.
• Patients with implants: confirm conditional status before the study.
• Patients with renal impairment: avoid gadolinium or use a group II agent.
• Patients with claustrophobia: wide bore systems, sedation, or PET-CT instead.
• Regions: PET-MRI is available in very few centres. PET-CT, MRI and CT are used separately.

Follow up tests.

• Biopsy of the most abnormal site, directed by the combined images.
• Repeat PET-MRI after treatment at an appropriate time.
• Diagnostic CT of the chest when lung nodules are a concern.
• Tumour markers and blood tests.
• Specialist neurophysiology (EEG) and neuropsychology tests in epilepsy and dementia.
• Cardiac biopsy or further cardiac MRI in suspected inflammatory cardiac disease.
• Referral to oncology, neurosurgery, cardiology or the relevant surgical team.